1. Field of the Invention
The present invention is directed to a liquid crystal cell having a liquid crystal layer between two carrier plates which have conductive coatings on a surface and at least one of the coatings is covered by an insulating layer of a silicon oxide base.
2. Prior Art
Since liquid crystal layers will decompose under DC voltage and drive processes for liquid crystal cells, which do not use DC voltage components, are virtually impossible, the electrodes must be additionally coated with a layer of material which is insulating in respect to DC voltage. A coating of this kind must fulfill a series of requirements. It must possess a high electrical resistance and be as pore-free as possible. Furthermore, a coating must be able to adhere to various types of materials such as the materials of the substrate and electrodes, within a relatively wide temperature range. Also, the coating must be mechanically, thermally and chemically stable and, furthermore, the layer or coating must transmit as much light as possible. Finally, the cost of providing the coating must not be too high. It would also be ideal if the insulating layer could produce a requisite rest state texture in the material of the liquid crystal layer.
Successful experiments have previously been carried out with SiO.sub.2 layers, and liquid crystal cells, which have conductive coatings covered by either a silicon oxide layer or silicon dioxide layer, are described in U.S. Pat. Nos. 3,700,306 and 3,932,026. Silicon dioxide is highly insulating, is highly suitable with regard to temperature changes and is extremely stable. However, without additional measures, the silicon dioxide orientates a liquid crystal, if at all, in a plate-parallel fashion which is known as a homogeneous orientation. If a plate perpendicular or homeotropic molecule alignment of the liquid crystal layer is required, as in the case of a dynamic scattering, the DAP effect or "phase change" effect, and if it were not desirable to dispense with the SiO.sub.2 layers which, in other respects, are highly suitable, specific silanol groups would have to be produced on the SiO.sub.2 layer surface and/or orientation means would have to be added to the material forming the liquid crystal layer. An article by Frederic J. Kahn, "Orientation Of Liquid Crystals By Surface Coupling Agents", Appl. Phys. Lett., Vol. 22, No. 8, Apr. 15, 1973, pages 386-388 discusses certain of such procedures for obtaining a desired orientation. A summary of the various orientation techniques is given in an article by L. A. Goodman, "Liquid-Crystal Displays--Packaging And Surface Treatments", RCA Review, Vol. 35, September 1974, pages 447-461. It is obvious that the processes mentioned above necessitate further production outlays and expenses and can only impair the quality of the liquid crystal. To this must also be added the fact that the double layer formed from SiO.sub.2 and the silanes can only be non-reflective by careful matching of the relevant layer thicknesses.